Python Notes

1. Write a program to find if a number is odd.

(Applying the modulo % operator and if-else branching):

# Program to check if a number is odd
num = int(input("Enter a number: "))

if num % 2 != 0:
    print(num, "is an Odd number")
else:
    print(num, "is an Even number")

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2. Write a program to find the largest among 3 numbers.

(Applying the if-elif-else ladder concept):

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = int(input("Enter third number: "))

if a >= b and a >= c:
    print("A is the largest")
elif b >= a and b >= c:
    print("B is the largest")
else:
    print("C is the largest")

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3. Explain classes and instances with a program (including concepts of objects and constructors).

• Class: A blueprint for creating objects.
• Instance (Object): An object created from a class.
• Constructor (__init__): A special method called automatically when an object is created. It is used to initialize the object's initial state (attributes).
• self parameter: Represents the specific instance of the class and allows access to its attributes.

class Student:
    # Constructor
    def __init__(self, name, age):
        self.name = name  # Initializing attributes
        self.age = age

# Creating an instance (object)
s1 = Student("Ravi", 20) 

# Accessing instance variables
print(s1.name, s1.age) 
# Output: Ravi 20

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4. Explain class and object with a program.

• Class: The blueprint.
• Object: The instance of the class.

class Student:
    def display(self):
        print("Student Class")

# Creating an Object
s1 = Student() 

# Calling a method using the object
s1.display()

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5. Explain the structure of a Python program with an example.

A Python program is a collection of statements executed by the Python interpreter in a top-down sequential manner. Unlike C or Java, Python does not require a compulsory main() function and relies heavily on indentation to define blocks of code.

A typical Python program consists of:

1. Import statements: Used to include modules.
2. Variable declarations and initializations.
3. Function definitions.
4. Executable (main) statements.

import math        # 1. Import section

x = 10             # 2. Variable declaration
y = 20

def add(a, b):     # 3. Function definition
    return a + b

result = add(x, y) # 4. Execution
print(result)

1. Exception Handling and re module

Part A: Exception Handling

An exception is a runtime error that disrupts program execution. Exception handling prevents the program from crashing, allows graceful error handling, and provides a better user experience.

It uses four main blocks:

• try: Contains the code that might throw an error.
• except: Contains the code that handles the error if it occurs.
• else: Executed only if no error occurs in the try block.
• finally: Always executed, regardless of whether an exception occurs or not (used for cleanup).

try:
    x = int(input("Enter number: "))
    print(10 / x)
except ZeroDivisionError:
    print("Division by zero")
except ValueError:
    print("Invalid input")
finally:
    print("Program ended")

Part B: Regular Expressions (re module)

A regular expression (regex) is a pattern used to match, search, or manipulate strings. Python provides the re module for handling these operations.

Common Functions:

• re.match(): Checks for a match only at the beginning of the string.
• re.search(): Scans the entire string for the first match.
• re.findall(): Returns a list containing all matches.
• re.sub(): Replaces occurrences of a pattern with a new string.

import re

# Finding all numbers in a string
pattern = r"\d+"
text = "Age is 20 and ID is 105"

print(re.findall(pattern, text)) 
# Output: ['20', '105']

2. Branching and Looping

Part A: Branching Statements

Branching allows a program to decide between alternative paths based on specific conditions. This forms the core of logical decision-making in Python.

• if: Executes a block if the condition is True.
• if–else: Two-way decision.
• if–elif–else: Multi-way decision (ladder).
• Nested if: An if statement inside another if statement.

a = 10
b = 20

if a > b:
    print("A is greater")
else:
    print("B is greater")
# Output: B is greater

Part B: Looping Statements

Loops avoid code repetition and allow a block of code to be executed repeatedly. They make programs efficient and scalable.

• for loop (Sequence-based): Used when the number of iterations is known. It iterates over a sequence (like a list or range).
• while loop (Condition-based): Used when the number of iterations is unknown. It executes as long as a condition remains True.

# For loop using range
for i in range(1, 6):
    print(i)
# Output: 1 2 3 4 5

Role in Problem Solving:

Together, branches (decision making) and loops (repetition) provide powerful control structures to solve real-world problems like factorial calculation, number classification, and searching.

# Example solving a problem (Factorial):
n = 5
fact = 1
for i in range(1, n+1):
    fact *= i  # Loop performs repetition
print("Factorial is:", fact) 
# Output: 120